The present invention relates to a method for measuring ocular metabolism and more particularly a method for measuring ocular oxidative metabolism using reflectance spectrophotometry.
Because the tissues in the interior of the eye are readily visible, conventional methods to detect diseases of the eye such as glaucoma, retinitis pigmentosa, ocular vascular occlusions or ocular tumors have been based on direct observation of the eye, direct or indirect measurements of functions such as visual acuity, electroretinography or visual evoked responses, or more invasive techniques such as tissue biopsy. Direct observation can only determine whether there are any anatomical structural changes. But the eye may appear to be normal and have oxygen related metabolic abnormalities which may lead to significant loss of function, such as is the case with diseases such as impending arterial occlusion and early glaucoma. Ocular tissue however, has not been heretofore subjected to metabolic monitoring as a predictor of ocular disease.
Reflectance spectrophotometry has been used to measure the presence of cytochrome c oxidase (hereinafter "cytochrome a,a.sub.3 ") such as is disclosed in Duckrow et al., Analytical Biochemistry 125:13-23, 1982. In the Duckrow method a reference and sample light beam having two separate wavelengths illuminates a tissue alternately. In Duckrow the tissue illuminated is the cerebral cortex of a rat. The light reflected by the tissue is detected and converted to an electronic signal and the signal associated with the reference wavelength is separated therefrom. However, this method is limited to the tissue region from which the externally applied light can penetrate and reflect to the detector. In addition, the optical signals that are derived are from relatively large areas of tissues rather than from small areas as would be desirable in the eye. Further, these methods have not been used in the eye because in order to optically detect cytochrome a,a.sub.3 through the front of the eye, measurements must take into account optical and metabolic contributions of photopigments, melanin, the cornea, and the lens.
Micro-light guides have been used in methods for measuring tissue fluorescence and reflectance of small areas of tissue as disclosed in Ji et al., American Journal Physiology, 236(3): C 144-156, 1979. This reference discloses use of this method in relation to rat liver and contains no discussion on its use in the eye.